Because communication underwater is very limited, PADI has established a universal set of signals to communicate the necessities of scuba diving. Here are a few of them:
The "OK" signal is the most easily recognized and possibly the most important of all the signals. It communicates that all is well and that the dive should continue. It is required that a diver entering the water from a bot signals OK to both the divemaster and to his or her diving buddy before descent can begin. On the surface, the OK signal is given by tapping your head with the thumb face of your fist. Divers should signal this motion to each other every few minutes to ensure that it is safe to proceed. It is also the most solid bond between a pair of divers in the buddy system.
Twisting a flat hand back and forth signals that something is wrong. This is a very important gesture because it could signal a shortage of air, failure or difficulty in equalizing, being uncomfortable, or any other problem a diver may encounter. This signal is usually immediately followed by another, more specific one describing the problem. In all training exercises pertaining to an unexpected problem, this motion is critical, and is probably the second most important signal in diving.
A common mistake that divers in training make is assuming that the "thumbs up" gesture means "OK". Rather, it means "ascend". This could be used routinely at the end of a dive, or it could signal an emergency ascent. Either way, it is not to be confused with the universal sign of being OK. Accordingly, the signal for "descend" is pointing the thumb downward. Both signals are used in every dive.
There are a variety of signals in scuba diving indicating levels of air left in the tank. The one shown at the right signals around 1500 PSI. On a regular dive, a full tank holds around 3000 PSI, which should last around half an hour. To be safe, though, anything below 500 PSI is considered risky, and a shallow dive usually ends at around that point. For a deeper dive, the blood nitrogen level is the primary indicator of when to end a dive.
Two finger pointed at the eyes means "look" or "watch". This signal is meant to draw the attention of other divers to either the signaler or to a specific location when followed by pointing in that direction. During the certification process, this signal seems to be used every other second. My instructor would want my focus locked on her at all times, even when there was a stingray swimming around behind her and I didn't know how to signal that yet.
In addition to controlling your level of air, it is an essential skill of any scuba diver to control the amount of time spent at a particular depth. Much of the primary training consists of learning how to read dive tables, which teach you about the nitrogen levels in your blood and the risks of too much buildup. Decompression illness, also known as "the bends", results from careless depth control or an uncontrolled ascent. To prevent this from happening, gestures like the "level off" one shown here are thoroughly instructed to an open water diver.